The value of ambulatory electrocardiography (AECG) in detecting pacemaker dysfunction before hospital discharge was assessed in 100 patients a mean of 1.2 days after pacemaker implantation. The incidence of permanent pacemaker dysfunction detected by AECG in the early postimplantation period, the frequency that pacemaker dysfunction detected by AECG was not detected by telemetric monitoring and the frequency that results of AECG led to pacemaker reprogramming before hospital discharge were determined. AECG detected at least 1 type of pacemaker dysfunction in 35% of patients and routine telemetry identified the abnormality in only 8% (p less than 0.001). Pacemaker dysfunction occurred in 42% of patients with dual-chamber devices and 27% of those with single-chamber devices (difference not significant). In the 35 patients who had pacemaker malfunction, a total of 50 instances of pacemaker dysfunction were detected. Failure of atrial capture occurred in 2% of patients, failure of atrial sensing in 9%, failure of atrial output in 1%, failure of ventricular capture in 8%, failure of ventricular sensing in 14%, failure of ventricular output due to myopotential inhibition in 11% and pacemaker-mediated tachycardia in 5%. The results of the AECG led to a clinical intervention in 22 patients (pacemaker reprogramming in 21 patients and lead repositioning in 1 patient) in whom no pacemaker dysfunction was suspected on the basis of telemetry or clinical symptoms. In conclusion, AECG provides additional benefit beyond that of routine telemetry monitoring in identifying pacemaker dysfunction in the early period after implantation.